This invention relates to a seal arrangement for sealing a rolling bearing having no seals and having a grease sealed in its inner space, and a spindle of a machine tool which is supported by the rolling bearing sealed by the seal arrangement.
Rotary shafts of various industrial machines, such as spindles of machine tools, are supported by a plurality of rolling bearings or a single double-row rolling bearing for bearing radial loads and a plurality of rolling bearings or a single double-row rolling bearing for bearing axial loads. The rolling bearings are typically arranged in a row in a bearing housing. If such different types of rolling bearings are arranged in a bearing housing in a row, the bearing housing may be provided with a front cover for pressing the front end of the foremost rolling bearing and/or spacers may be disposed between adjacent bearings and/or between the rear end face of the rearmost bearing and the rear end wall of the bearing housing.
Rolling bearings for supporting rotary shafts of industrial machines have a grease sealed in their internal space. In some machine tools that need run-in before a high-speed normal run, such as lathes, excess grease is sealed in the bearing inner space before run-in. After run-in, any excess grease not forming a thin film on the internal surfaces of the bearing is removed because such unstable grease, which is present in relatively large masses in the bearing inner space, tends to get stuck between rolling elements or between rolling elements and other bearing members all at once during a normal run of the bearing, thereby suddenly increasing the resistance to the rolling motion of the rolling elements during a high-speed normal run of the bearing. This may lead to abnormal temperature rise in the bearing.
The amount of grease that forms the thin film on the bearing internal surfaces after run-in is no more than 10% of the volume of the bearing internal space. The grease forming the thin film after run-in is gradually supplied to the rolling surfaces of the rolling elements during a high-speed normal run of the bearing. But because the initial amount of grease forming the thin film after run-in is relatively small as mentioned above, the supply of grease to the rolling surfaces tends to run short soon. This may shorten the life of the bearing.
As a countermeasure for this problem, many conventional ball bearings use a seal mounted to the outer ring. With this arrangement, the inner surface of the seal adds to the total internal surface area of the bearing on which the grease forms the thin film, thereby increasing the initial amount of grease sealed in the bearing after run-in. The seal is mounted to the outer ring so that the seal also serves to prevent leakage of grease, which tends to move radially outwardly under centrifugal force during a normal run of the bearing.
Some conventional cylindrical roller bearings also have a similar seal mounted to the outer ring (as disclosed in JP utility model publication 7-46815). But practically, such cylindrical roller bearings are extremely unpopular. This is because an end user has to separate the outer ring of a cylindrical roller bearing to adjust the radial bearing clearance before assembling. A seal mounted to the outer ring therefore tends to be a major obstacle. Moreover, such a seal tends to be damaged during transportation due to displacement of the bearing parts.
It is therefore acutely desired and is an object of the present invention to increase the initial amount of grease sealed in the bearing internal space without the need to mount a seal to the outer ring, thereby prolonging the bearing life.